Abstract Epidermal growth factor receptor (EGFR) has been heavily exploited as a target for blockade in cancer therapy. In normal epithelial cells, ligand-induced internalization of activated receptors and their sorting for degradation in lysosomes is the limiting mechanism for EGFR signaling. This desensitization process is commonly circumvented by cancer cells to promote their growth and survival even in the presence of EGFR signaling inhibitors. We have identified a novel role for the sterol biosynthesis pathway to influence this recycling process which could significantly improve the efficacy of EGFR-targeting inhibitors. Silencing of SC4MOL (sterol C4-methyl oxidase-like) significantly sensitizes tumor cells to EGFR inhibitors using a network-guided siRNA-based screen (Astsaturov, 2010). We have determined that SC4MOL and a functionally linked partner protein, NSDHL (NADP-dependent steroid dehydrogenase-like), are negative regulators of trafficking of EGFR and its family members ErbB2 and ErbB3 from the plasma membrane to the lysosome for destruction. Our central hypothesis is that metabolic blockade of the sterol biosynthesis pathway will accelerate receptor degradation and thus suppress EGFR signaling in vitro; in NSDHL conditional knockout mice, this will limit epithelial carcinogenesis. Our immediate objective is to validate a new metabolic target for cancer therapy involving these previously unexplored genes in the distal sterol biosynthesis pathway. With the strong team of collaborators assembled, we propose the following 3 specific Aims: Aim 1. Investigate the mechanism of EGFR signaling regulation by genes in the sterol biosynthesis pathway. On the basis of preliminary data and complementary bioinformatic analysis, we hypothesize that blockade of SC4MOL and NSDHL causes altered EGFR trafficking that accelerates EGFR degradation in lysosomes. Aim 2. Determine the value of combined targeting of SC4MOL and EGFR in tumor xenografts. Based on our preliminary in vitro data, we propose that shRNA silencing of SC4MOL will increase the response of tumor xenografts to EGFR blockade. Aim 3. Investigate in vivo effects of sterol pathway on EGFR signaling and susceptibility to carcinogens. We hypothesize that the EGFR-antagonistic effects of the epithelial NSDHL deficiency will limit the H-Ras-dependent or -independent carcinogenesis. This proposal is significant because it will provide fundamentally new knowledge on how the metabolism of sterols regulates signaling activity of essential cancer receptors such as EGFR. We believe that pharmacological inhibition of sterol pathway targets such as SC4MOL and NSDHL has the potential for cancer chemotherapy and chemoprevention as an entirely novel class of agents.